Help guessing what this is

[RyKenna]

That little green spot on the rock it closes up also and the video is the tank.

 

[BeanAnimal]

Looks like a baby Mojano anemone…. Kill it.

 

[ISpeakForTheSeas]

I'm late, but it could also be a Corallimorph (a Ball Anemone, which is actually related to soft mushroom corals).